This invention relates to an electric thermal unit for controllably heating cylinders in extruders and drawing machines for plastics, rubbers, and the like, which unit is so designed and arranged as to define therein a double air circulating interspace.
It is a known fact that some plastomeric and elastomeric material processing operations involve the use of specific equipment (such as extruders, injection presses, and drawing machines) comprising essentially a screw set for rotation within a coaxial cylinder with a limited amount of play.
Also known is that said cylinder must be provided with a heating means controlled by appropriate automatic control systems and effective to suitably soften the material preparatory to its being forced through a forming die (or die plate).
Such heating means conventionally comprises a spiral coil resistance heater which, being laid between suitably shaped ceramic supports, encircles the cylinder.
More specifically, such prior resistance heaters, and related insulating supports, are attached to a ring-like structure formed in two or more parts pivotally connected to one another and being adapted for installation in close contact relationship with the outer surface of said cylinder.
It should be pointed out, however, that said ring-like structure accommodating the resistance heaters undergoes itself an undesired surface heating effect, and may constitute a potential risk for the machine operators.
Furthermore, owing to the natural tendency of heat to flow upwards, the cylinder temperature is at all times slightly above that of the bottom region. This circumstance may originate non-homogeneous areas in the mixture, thereby the finished products may fall short of expectations.
It should be also remembered that conventional electric thermal units are sometimes equipped with a cooling system which can favor a cylinder temperature maintained within suitable ranges for the intended processing.
Such prior systems generally comprise a jacket enclosing the electric thermal unit wherethrough air is circulated which is only intended to provide local cooling, and is accordingly apt to enhance any temperature differential between the upper and lower regions of the extruder cylinder.